1. Field of the Invention:
The invention relates to inverter circuits and in particular to high power, high frequency resonant inverter circuits of reduced size especially useful in airborne systems.
2. Description of the Prior Art:
It is desirable to reduce the size of power converters used as power supplies for modern electronic systems. In general, although progress has been made in increasing power densities and therefore reducing size, the degree of miniaturization of power converters has been much slower than for electronic systems.
Among the major problems associated with building a high power density supply are power dissipation, noise generation and control. It has been found that an increase in the frequency of power conversion allows for a reduction of the size of power supplies. However, power dissipation results from increased switching conduction eddy current and drive losses associated with high frequency operation. Noise generation becomes more severe as parasitic conductances and capacitance become more significant at the higher operating frequencies. Further the high frequency designs are also more sensitive to device operation parameters. The miniaturized size limits volume available for energy storage and thereby excludes some traditional linear control methods. Downsizing also results in smaller thermal area available for heat removal.
Some recent developments in converters employing resonant or quasi-resonant components have attempted to address problems associated with power dissipation and noise generation. In particular, the effects of the parasitic elements have been incorporated into the design making use of them as resonant elements, although of lower quality than desired. As a result, switching losses and noise generation can be somewhat reduced by limiting the rise times of switching waveforms. Control of these quasi-resonant devices, however, is difficult under severe load transients which are to be expected.
A particular series/parallel resonant topology has been employed in high power inverters which use silicon control rectifiers (SCRs) as switches. Operation frequencies have generally been limited to below 20 KHz by the SCR recovery time of about 10 microseconds. Further SCRs have a relatively low maximum operational temperature which has restricted SCR resonant inverters to relatively large surface based applications.
In airborne systems, were size restrictions are more stringent, square wave converters are often used. These devices operate at higher frequencies, lower peak currents and higher temperatures due to the use of transistors for example instead of thyristors. The systems also generally do not use resonant components and are thus reduced in size. Square wave converters have disadvantages including higher switching losses, increased electromagnetic interference caused by sharp switching transitions, shoot-through or short circuit vulnerability and complicated control characteristics.